Burner safety control



Oct. 17, 1944.

R. E. YATES 2,360,532

BURNER SAFETY CONTROL Filed Aug. 21, 1942 2 Slieets-Sheet 1 Oct. 17, 1944. Y R. E. YATE-S, v 2,360,532

BURNER SAFETY CONTROL Filed Aug. 21, 1942 2 Sheets-Sheet 2 ATV Patented Oct. 17, 1944 BURNER SAFETY CONTROL Robert E. Yates, Chicago, Ill., assignor to Drying Systems, Inc., Chicago, 111., a corporation of Illinois Application August 21, 1942, Serial No. 455,560

9 Claims.

This invention relates to fuel burning systems and particularly to safety control means therefor.

Where fuel burners are used in heating,baking, drying or like systems, many hazards may arise when unburned fuel accumulates in the system due to ignition failure, combustion failure or the like, for under such circumstances a danger of explosion or other serious damage is created.

Systems or installations of the aforesaid character are therefore customarily equipped with safety control means operable in response to failure or absence of combustion to automatically shut off the supply of fuel to the burner or burners.

Numerous types of safety control means or apparatus of the aforesaid character have been heretofore proposed. The earlier forms of such apparatus entailed the use of heat-sensitive devices 'such as thermostats, thermocouples, pressure diaphragms and the like which were indirectly responsive to the presence or absence of a flame in a burner for effecting whatever control operations were necessary with respect to the burner. Such devices were generally unreliable because an appreciable time lag was entailed in the operation thereof and such devices were readily affected by disturbances such as external air drafts which had no direct relation to combustion conditions in the burner; Later forms of flame detectors have operated in dependence upon the flow of electrical current across a gap between a pair of spaced electrodes that are arranged to be bridged by the flame of the burner. Usually one of these electrodes constitutes what is known in the art as a flame rod which is disposed in the flame zone of the burner so as to be contacted by the flame when such a flame is present, while the other electrode may consist of another flame rod similarly situated or it may be afforded by a portion of the burner structure itself. Flame detectors of this character may be roughly divided into several classes.

One class of such apparatus entails an arrangement in which the value of an ohmic resistance of the electrical path between the electrodes is the determinin factor in the operation of the apparatus, the magnitude of this resistance being governed by the presence or absence of flame in the burner. This class in turn may be said to comprise at least two varieties. In one of these the electrical current flow through the flame path is produced by impressing a relatively high voltage across the flame-sensing electrodes, this voltage being sufflcient to set up a current having a great enough magnitude (provided a flame is present) to operate a relay or the like. In the other form of this apparatus an altemating-current voltage is impressed upon the flame-sensing circuit and an electron tube means is arranged to be governed not only according to the magnitude of the current flow through the flame gap, but also in response to the phase relation of this current with respect to the impressed voltage.

An example of the last-mentioned apparatus is disclosed in my Patent No. 2,282,551, patented May 12, 1942, and one of its outstanding advantages is that it operates to detect not only flame failure but also an unsafe condition which may arise when the flame gap is accidentally shortcircuited.

Another of the classes of flame detectors to which reference has been made hereinabove entails an arrangement in which the conduction of electrical current throughthe flame is made to depend not so much upon the conductivity of the flame itself as upon the electron-emitting properties of an electrode which is arranged to be heated by the flame. Such flame detectors tend to be unduly sensitive and moreover are unsatisfactory when used on alternating-current because of the fact that the flame itself has a current-rectifying effect which tends to block the unidirectional flow of'current set up by thermionic emission, unless the electrodes are so arranged that the emitted electrons travel from the hot to the cool electrode substantially in the direction of flame propagation (that is, away from the burner nozzle). It is disadvantageous to'arrange the electrodes in this manner, however, because the cool electrode must thenibe placed outside of, but very close to, the edge of the flame and any fluctuations of the flame due to crossdrafts or the like may produce corresponding fluctuations in the flow of electron current, with detrimental effects upon the accuracy of operation.

Still another class of flame detecting safety devices is that wherein the arrangement makes use of the substantially unilateral conductivity of a flame without relying upon any thermionic emission characteristics of an electrode, and examples of this class of flame detectors are illustrated and described in my copending application Serial No. 449,827, filed July 4, 1942, and in the copending application of Richardson and Yates, Serial No. 400,332, filed June 28, 1941. Because of the greater ease with which a current of electrons or negative ions may flow in the direction of flame propagation than contrary thereto, a substantially unidirectional flow of negative ions or electron current takes place between the spaced electrodes in a direction away from the burner nozzle when an alternating-current voltage is impressed across these electrodes. This unidirectional or rectified current is utilized to charge a dielectric body such as a condenser to thereby control the potential applied to the control element or grid of an electron tube and thus regulate the amount of current flow in an output circuit of the tube. If the flame is extinguished or becomes too low, the rectified current flow ceases and the condenser loses its charge, the effect of this being to so condition the electron tube circuit that a control operation is effected or a warning signal given,

The safety control devices of the aforesaid copending applications Serial No. 400,332 and Serial No. 449,827 are so constructed and arranged as to take into consideration the possibility that the flame-sensing means itself may become defective in several respects and thereby cease to afford protection for the burner system. For example, a flame-sensing rod or electrode may become softened due to the heat of the flame and may therefore so bend thatit engages a part of the burner. This is dangerous because the flame is short-circulted and is therefore no longer included in the input circuit of the electron tube, so that the apparatus is adapted to function safely upon the occurrence of such a contingency. Then again, instead of there being a direct electrical contact of the flame rod with the burner there may be an accumulation of hygroscopic, carbonaceous or other electrically conductive substance extending between the flame rod and the burner which has a leakage or shunting effect that may simulate a flame even when there is no combustion of the fuel in the burner. It is likewise possible that sufficient leakage may take place through the insulation surrounding the conductors in the flame-sensing circuit to produce a conditon simulating the presence of flame in the burners In some prior control systems efforts have been made to check leakage effects such as have just been described in a number of ways, as by providing one of the flame-sensing electrodes with a sleeve that is intended to intercept any leakage current passing between the electrodes,

by a path other than through the burner flame, and upon the occurrence of excess leakage from an electrode to such a sleeve the apparatus is intended to react as though there-had been a flame failure. Such an arrangement is unsatisfactory because obviously the sleeve cannot extend into the flame zone since it would then prevent the proper operation of the device, and hence it is ineffective to detect leakage paths in this region.

Other prior devices of the character which function in dependence upon the ohmic resistance of the flame path-have dispensed with the special sleeve on the flame-sensing electrode, and it has been intended in certain of these devices that they are to react safely whenever there is any substantial leakage from an electrode other than the normal flow of current through the flame, whether this exists by reason of short-cir cuiting, insulation failure or carbonization of the electrode in the region of the flame itself. In these prior devices an all-around leaka e check of the flame-sensing'circuit is made as a condition precedent to operation of the system. How

ever, once the. operation of the burners has been initiated, the safety apparatus thereafter functions to detect only flame failure or the existence of a low-resistance shunt path between the flame electrodes, and it will not operate safely upon flame failure if a leakage path having a resistance comparable with thatof a normal flame path is established between the flame electrodes in the region of the flame while the burner is in operation. Usually in such burner systems a safe reaction of the control apparatus in response to the detection of current leakage conditions in a burner does not occur until the system has been recycled, that is, disabled due to the operation of some other means such as a room thermostat and then started up again, and if such recycling does not occur soon enough, a risk of explosion is created.

The safety control devices illustrated in the two aforesaid copending applications are so constructed and arranged as to prevent misoperation of the system under all of the circumstances enumerated above, and an important object of the present invention is to accomplish these results in an improved manner such that manufacture, installation and maintenance of the control apparatus may be simplified and facilitated.

The control apparatus illustrated in my aforesaid copending application Serial No, 449,827 is such that the electron tubes embodied in the apparatus are checked as to conductivity and operativeness prior to the initiation of the operation of the fuel burning system and during the entire time that the system is in operation, and this is accomplished by rendering an electron tube therein conductive in response to the sensing of a flame in a burner and nonconductive when there is a failure of combustion or upon establishment of a short circuit or leakage path through the flame gap, and. in this way any internal failure of the tube which causes it to become perma nentl nonconductive immediately produces the same reaction of the control apparatus as though any of the aforesaid unsafe conditions had been detected. In attaining this end in the control apparatus of my aforesaid copending application a normal negative bias on the control grid of'the electron tube has been utilized to render the tube normally non-conductive and upon detection of a flame this normal negative bias has been in effect reduced by applying an offsettingv be attained in control apparatus which may be more easily constructed and maintained A still further object isto enable such safety control apparatus tobe used in multiple burner systems in a novel manner without unnecessary duplication of parts and at the same time to afford the features and advantages described hereinabove.

Other and further objects of the present invention will be apparent from the following descrlpt n and claims and will be understood by reference to the accompanying drawings which.

by way of illustration, show preferred embodi- 1 ments and the principle thereof and what I now but illlustrating one manner in which the invention may be adapted for use in a multiple burner system.

Referring first to Fig. 1, wherein the basic construction is shown, it will be observed that the apparatus is adapted to control the operation of a burner generally designated l2 which is supplied with fuel through a pipe l3 that is grounded as indicated at It. A self-closing fuel valve 15 having an operating solenoid I6 is embodied in the fuel supply pipe i3 to prevent the supply of fuel to the burner l2 when the solenoid i6 is in a deenergized condition. The burner I2 is equipped with a flame-sensing rod or electrode H which is insulatedly mounted in such a manner as to extend into the flame F of the burner when such a flame is present, the electrode ll being maintained in predetermined spaced relation with the tip or nozzle of the burner I2. The electrode I1 is preferably made of a heat-resisting, electrically conductive material having substantially no thermionic emissivity. In the present instance the burner i2, being grounded at M, constitutes the other electrode; however, it will be understood that a secondrod such as ll positioned closer to the nozzle of the burner than the first rod I! may be employed equally well for this purpose. I The control apparatus constituting the preferred form of the present invention includes an electron tube 18 which is diagrammatically represented in Fig. l as being of the tetrode type, having a cathode IS, an anode 20, a screen grid 2i and a control grid 22. While other types of tubes may be utilized, I preferto employ a gasfilled electron tube l8 in order to afford a trigger action, but it is to be understood that I do not limit myself to such a tube since a vacuum tube may be utilized in lieu thereof if desired.

Alternating current is supplied to the apparatus by a suitable source such as transformer 50 having a primary winding connected at its opposite ends by wires 52 and 53 respectively to line wires LI and L2. The transformer 50 has a secondary winding 54 provided with three taps 55, 56 and 51, the tap it being electrically intermediate the taps 55 and 51. One end of the transformerprimary winding 5| is grounded as at 52' for purposes which will be explained hereinafter, this ground connection being disposed in an electrical sense at that end of the primary winding 5i which corresponds with the location of the tap 51 in the secondary winding 54 The ground 52 is in practice located at the electrical service entrance which may be a considerable distance from the transformer 50, and hence the location of the ground 52' as herein illustrated is merely diagrammatic. The cathode I! of the tube It is connected by conductors 59 and 60 in series to the tap 56, while the anode 22 of the tube I8 is connected through the winding of a relay i toa conductor 62 leading to the tap 55. The relay 6| has a contact 64 which is normally open when the relay is deenergized and which closes when the relay is energized to complete a circuit for energizing the fuel valve solenoid I6.

The valve actuating solenoid [6 has one side thereof connected to a line wire L3, while the other side of the solenoid is connected to the other line wire L4 by wires 63 and 63' and a normally open push button starting switch 65 so that by manual closure of the switch 65 the valve oper- .ating solenoid 16 may be energized, to thereby open the fuel valve l5 and permit initial flow of fuel to the burner l2. The movable relay contact 64 is connected by a wire 66 at juncture of the wires 63 and 63', while the stationary contact 61 of the relay 6| is connected by a wire 61 to the line wire L4, and hence after the manual switch 55 has been opened at the end of a burner starting operation, the valve operating solenoid I6 is controlled solely by the relay contact 64.

The screen grid 2| is connected by a wire 68, a protective resistor 69 and a Wire 10 in series to the transformer tap 51, and through this arrangement the grid 2! is biased negatively with respect to the cathode I9 so as to normally render the tube [8 non-conductive. The filament H of the tube i8 is connected by wires 12 and 13 to the wires 59 and 10 respectively, and is thus supplied with the requisite filament voltage.

The flame-sensing electrode i! is connected by a conductor 15 to a resistor 16 and thence through this resistor to the conductor H connected to the conductor 59 so as to be-thereby connected to the transformer tap 55. A protec- 4 ,ductors 8| and 82 in series to the line wire Li.

A condenser 83 has a wire 84 connecting one side thereof to the conductor 11 and on its other side the condenser 83 is connected by a wire 85 to the conductor 82. The control grid 22 of the tube i8 is connected by a wire 86, a protective resistor 81 and a wire 88 in series to the wire 85 so as to be thereby'connected to one side of the condenser 83.

It has been explained hereinabove that the voltage induced in that section of the transformer secondary 54 between the taps 58 and 51 affords a negative bias for the screen grid 2i of the tube I8 during the forward half-cycles of the alternating current when the anode 22 is at a positive potential with respect to the cathode I9, and this negative bias tends to render the tube i8 non-conductive. It has also been explained hereinabove that a flame as F in a fuel burner as [2 has the property of rectifying alternating current; that is, during those half-cycles in which the burner i2 is negative with respect to the electrode i1, electrons or negative ions may flow from the burner l2 to the electrode I! in the direction of flame propagation, provided a flame as F is present. However, during the intervening half-cycles substantially no current flow can take place through the flame F between the electrode l1 and the burner l2.

As the apparatus is arranged in Fig. 1 the tube l8, in so far as the bias between anode and cathode is concerned, tends to be conductive in those half-cycles of the alternating current in which .the anode 22 is positive with respect to the be conductive in such forward half-cycles of the alternating current unless the blocking action caused .by this negative bias of the screen grid 2I is rendered ineffective in response to the presence of a flame F, as will be hereinafter described. During such forward half-cycles of the alternating current it will be evident that due to the ground connections I4 and 52' the electrode I? is positive with respect to the burner I2, and hence electrons or negative ions may flow from the burner to the electrode in the direction of flame propagation, provided a flame is present. However, during the intervening or inverse halfcycles of the alternating current the electrode I1 is negative with respect to the burner I2, and therefore the flame F is effective to block the flow of electron current between the burner I2 and the electrod I'I. Thus, when a flame F is present in the burner I2, the current flows through the flame-sensing circuit during forward half-cycles of the alternating current, this circuit extending from the line wire LI through wires 82 and 85, the condenser 83, the wire 84, the resistor I6, the conductor I5, the electrode II, the flame F, the burner I2, the ground connections I4 and 52' and the conductor 52 to line wire L2. Due to the rectifying eifect of the flame F, this current is of a non-bilateral character so that the condenser 83 assumes a positive charge which in the course of a relatively few pulsations of the alternating current reaches such a magnitude that the control grid 22 is thereby charged positively in an amount sufficient to overcome the blocking effect of the normal negative bias on the screen grid 2I. long as the flow of rectified current continues in the flame-sensing circuit this positive charge on the grid 22 renders the tube I8 conductive during the forward halt cycles of the alternating current, and a pulsating current is passed through the winding of the relay 20 to maintain this relay energized, and the relay contact 64, therefore, remains closed and maintains the fuel valve solenoid I8 energized to thereby enable fuel to be supplied to the burner I2.

In the event that the flame F in the burner I2 is accidentally extinguished, the flow of rectifled current through the aforesaid flame-sensing circuit ceases and the charge on the condenser 83 leaks ofi relatively rapidly' through one or more electrodes of the tube I8. -Since the positive bias on the grid 22 is thus removed, the normal negative bias on the other grid 2| again becomes effective, and the tube I8 ceases to conduct current and the relay 20 deenergizes to open its contact 84. This results in deenergization of the solenoid I8 and consequent closure of the fuel valve I5 to prevent emission of unburned fuel from the burner I2.

If the flame electrode I! accidentally becomes grounded as by engaging the burner I2, it will be recognized that the grounded electrode provides a leakage path along which the positive charge of the condenser 83 may immediately be dissipated; and upon such removal of the positive bias from the control grid 22, the tube I8 returns to its non-conductive state so that the relay GI is deenergized. Hence the fuel valve I5 closes. While the electrode I1 is thus grounded, alternating current continues to pass through the aforesaid flame detecting circuit which, of

course, includes the condenser 83, but under such circumstances the control grid 22, although it is charged slightly positive relative to the cathode I8 during the forward cycles of the alternating current, does not become sufficiently positive to overcome the blocking action of the screen grid 2I. In attaining this end it should be observed that the resistance of the resistor 16 should be made relatively large with respect to the alternating current resistance of the condenser 83, thereby to insure that the potential difference between the points 85' and 84' in the flame detecting circuit is. relatively small. The relationship should of course be such that the potential difference between points 84' and 88',

with respect to the condenser 83 through the the conduit 90 is grounded as at 8| so that failure of the insulation between the sheath l8 and the conduit 80 will effectively shunt the condenser 83 and cause shut down of the system as aforesaid.

When operation of the burner I2 is to be initiated after it has been disabled by the safety the fuel has been properly ignited, he releases the,

starting switch and thereafter the energizing circuit for the fuel control valve solenoid I6 is maintained closed through the contact 84 of the relay GI (the tube I8 having been rendered conductive as hereinbefore described upon the establishment of a flame F in the burner I2) unless and until a hazardous condtion again arises requiring a shut down of the system.

It has been stated hereinabove that the present invention may be readily adapted for use in multiple burner systems, and one such arrangement for achieving this object is diagrammatically illustrated in Fig. 2; Thus, there is provided a master control unit designated MU which is sub. stantially identical with the device shown in Fig. 1, and which is operstively associated with one of the fuel burners I2 included in a bank of burners that are supplied with fuel by a single pipe II having the necessary branch connections to the various burners. Corresponding parts of the structureshown in Figs. 1 and 2 are duigare provided a plurality of secondary control units designated SU which are respectively associated with the other burners |2a of the system in much the same manner as the master unit MU is associated with the burner I2. Thus, each burner [2a has an electrode Ila which is connected by a shielded conductor 15a to a resistor Mia to the control to a condenser 83a so as to form part of the flame detecting circuit in the respective secondary unit SU. The shielded conductors l and Ma may be passed through individual grounded conduits or it may be preferred to run these conductors through a common conduit for at least a portion of the respective lengths. The secondary units are substantially identical in construction with the master unit MU except that the relays as Ma, have their relay contacts connected in series, as will be presently described, to jointly govern the operation of the solenoid l6 of the normally closed fuel valve l5 which controlsthe fuel supply to all of the burners l2, and hence corresponding parts of the master and secondary control units bear similar reference characters which differ only in the sufiixes thereof. The anodes 22a of the tubes Na in the several secondary units SU are connected through the coils of their respective relays 6 la to the taps 55a of their respective transformer secondaries 54a in the same manner as in the master control unit MU. In like manner the cathode iii of the tube l8 and the cathodes 19a of the tubes Eta are all electrically connected by wires 59 and 60, respectively, to the taps 56 of their respective transformer secondaries 54 and M a. Hence, the electron tubes l3 and Wu in the various control units are connected and operate in the same manner as in the embodiment of the invention shown in Fig. 1, and anode current of these tubes passes through the windings of their respective relays 6i and lila to close the contacts lit and Ma thereof in response to the presence of a flame at their respective burners l2 and lfa. The wire 51 connects the line wire L l to the stationary contact 6'! of the relay 6!, a wire 93 connects the relay contact 5 1. to the stationary contact of the relay 6m of the first secondary unit SU, a wire 94 connects the relay contact Ma of this secondary unit SU to the stationary contact of the relay Bla of the next unit SU, and so on until the last of the units SU has been thus connected, and the contact 64a of the last unit SU is connected by a wire 96 to one terminal of the solenoid l6; and since the other terminal of the solenoid i6 is connected to the other line wire L3, the solenoid I6 is controlled by a series circuit which may be broken by opening of any one of the relay contacts 64 or Ma.

It will be recalled that the tubes i8 are normally biased so as to be nonconductive, and that when a flame as F is present in a burner as I2 the associated tube as I8 is rendered conductive. Similarly, the tubes lBa. are normally biased so as to be non-conductive and when the flames are present in all of the other burners [2a the corresponding tubes l8a are all conductive, and hence the relays 6| and Bla are energized when a flame is present at all of the burners. However, if a flame in any one of the burners becomes extinguished, or ,too low, the tube l8 or l8a associated with such burner is rendered non-conductive so that current flow stops in the associated relay coil, which relay thereupon deenergizes and opens its contact 64 or 64a. The fuel supply pipe l3 has a valve I5 controlled by the solenoid 16 which receives its energizing current through the relay contacts 64 and 64a of all of the relays BI and Bla in series, and when a relay Si or 61a deenergizes to open the contact 64 or 64a thereof the solenoid l6 deenergizes and enables the fuel valve IE to close and thereby interrupt the supply of fuel to all of the burners in the system. i

The other safety features described hereinabove in connection with-the circuit of Fig. lare also afforded in the case of each and every control unit in the multiple system as shown in Fig. 2. Thus, if there is a shunting of the flame gap in any particular burner due to grounding out of the flame-sensing electrode or the building up of a non-rectifying resistance path across the flame gap as a result of carbonization and the like, the electron tube in the control unit associated with that burner is rendered nonconductive so as to ,deenergize its relay and thereby break the circuit to the fuel valve solenoid and shut off the supply of fuel. The same is true if there is an insulation failure which causes excessive leakage in the cable including a conductor IE or (5:1 leading to the flame electrode of a particular burner. Hence, a dependable safety control apparatus for a multiple burner system is afforded.

From the foregoing description it will be apparent that the invention provides a safety control apparatus of novel and economical construction and which is more reliable in operation than the control devices heretofore employed to govern fuel burner systems and the like. Thus, the present apparatus acts to protect the burner system against hazards which arise not only when the fuel admitted to a burner or burners is improperly ignited resulting in a failure of combustion, but also when the safety control apparatus itself becomes defective in any of a number of different ways. For example, where the apparatus is adapted to respond to the flow of elec trical current through a gap between spaced electrodes positioned in the region of a flame in a burner, as is done in the present instance, there is always the possibility that the aforesaid electrical path through the flame may be accidentally shunted due to a flame-sensing electrode coming into electrical contact with the other electrically conductive substances such as carbon accumulating on a flame-sensing electride or on the surface of the insulators supporting such electrode, and in some instances the ohmic resistance of this shunt path may be of a value comparable with that of a normal flame path so that the gap between the electrodes has substantially the same conductivity (except that it is of bilateral rather than unilateral character) in the absence of a flame as would normally be the case when a flame is present. The present invention is adapted to guard against dangers attending such improper conditions in the burner and is particularly useful and advantageous in that all of these safety features are afforded in the same apparatus without unduly complicating the construction thereof.

The present invention utilizes the unilateral electrical characteristics of flames to control the operation of electron tubes for the purpose of producing a safe response upon the occurrence of any of the undesirable contingencies mentioned above, and the arrangement is such that an electron tube is rendered conductive when a flame is present in the burner and nonconductive if the flame is extinguished or the electrical path through the flame is shunted so as to nullify the current-rectifying effect of the flame. The present invention attains the desired controlling action in a simple and expeditious manner which facilitates manufacture as well as repair a maintainance of the system, for the normal biasing ii of the control tube is accomplished by the use of the screen grid of the tube while the desired conductivity of the tube is attained by a bias impressed on a second and independent control grid.

So long as the tube is maintained in a continu- 10 ously conductive condition fuel may be admitted to the burner, but if there is a failure of combustion or the flame path becomes shunted, the condenser which provides the positive charge on the second control grid of the tube loses its unidirecl5 tional charge and; the electron tube is then rendered nonconductive. The supply of fuel to the burner is thereupon interrupted so that unburned fuel cannot be emitted by the burner. It

will be understood, of course, that any other or additional control operations with respect to the fuel burner may be performed by the apparatus when an unsafe condition exists, such as operating an alarm or automatically operating an ignition means to reignite the fuel, and it will also be apparent that the present apparatus may be used in cooperation with other control devices such as limit switches, timing mechanism and the like.

While the apparatus of the present invention has been found to be particularly applicable to 3" single burner installations, still another feature of the present invention is that it may be readily adapted to use in connection with multiple burner systems as well as with single burners. In

multiple burner operation unnecessary duplication of fuel valves is avoided by providing a single master control unit to control a common fuel supply means, and a plurality of secondary con-. trol units each operatively associated with a respective burner, each of said control unitsm responding to conditions in its burner to govern the common fuel supply means.

Still another advantage of the present invention is that if the insulation surrounding the conductors in a flame-sensing circuit becomes faulty so as to enable excessive-leakage of current to take place through the insulation, the apparatus is able to detect such conditions and respond in a safe manner. This is particularly important where the apparatus is to be utilized in chemical plants or the like where the insulation is sub- Ject to damage from corrosion or other causes.

While I have illustrated and described selected embodiments of my invention, it is to be understood that these are capable of variation and Y 8 1. In a device for detecting the presence of a 0 flame in a fuel burner or the like, an electron tube having a pair of control elements, means normally biasing one of said control elements to render said tube nonconductive, a condenser con- 35 nected to the other of said control elements and effective when charged unidirectionally in a given sense to apply an opposite bias on said other control element and thereby render said tube. conductive, andmeans providing a circuit extending from an alternating current source through a path in said flame of substantially unilateral conductivity and through said condenser whereby said condenser tends to become charged unidirectionally for rendering said tube conductive 7 when a flame is present, said circuit embodying a resistor in series with said condenser and operable when said flame path is short circuited to maintain the instantaneous alternating current potential at said condenser below the level required to render said tube conductive.

2. In a flame detector operating in dependence upon the property of a, flame to conduct electrical current between two spaced electrodes more readily when the first of said electrodes is positive with respect to the second electrode than when the relative polarities of said electrodes are reversed, the combinationof an alternating current transformer having three taps, the potential of the second tap being intermediate the potentials of the first and third taps, an electron tube having an anode, a cathode and a pair of control grids, means electrically connecting said anodeto said first tap, means electrically connecting said cathode to said second tap, means ineluding a protective resistor connecting one of said control grids to said third tap to thereby apply a bias to said one control grid which renders said tube normally non-conductive, a condenser having one side thereof electrically connected to the other of said control grids and adapted when charged to at least a predetermined potential in a predetermined sense to bias said other control grid to overcome said one control grid and thereby render said tube conductive, and circuit means including said condenser, a resistor connected in series with the other side of said condenser, and a source of alternating current and the electrical path through the flame between said electrodes.

3. In a device for detecting the presence of a flame in a fuel burner or the like, an electron tube having an anode, a cathode and at least two control grids, means normally biasing said anode and cathode and a first one of said control grids to render said tube nonconductive, a condenser connected onone side to a second one of said control grids and effective when charged unidirectionally'in a predetermined amount to nullify the blocking action of said first control grid and thereby said tube conductive, and means providing a circuit extending from an alternating current source through a path in said flame of substantially unilateral conductivity and through said condenser whereby said condenser tends to become charged unidirectionally for rendering said tube conductive when a flame is present, and said circuit embodying a branch lead from the other side of said condenser to said cathode through which said condenser loses its unidirectional charge and is rendered ineffective when said circuit is interrupted due to flame failure and said circuit also including resistance means connected to said other side of said condenser and operable when said circuit becomes bilaterally conductive due to shunting of the flame path to maintain the instantaneous positive potential of said second grid below the value required to render said tube conductive whereby said tube is rendered non-conductive when said circuit is bilaterally conductive.

4. In a device for detecting the presence of a flame in a fuel burner or the like, an electron tube having an anode, a cathode and at least two control grids, means normally biasing said anode, cathode and a first oneof said control grids to block said tube and render the same nonconductive, a condenser connected on one side to a second one of said icontrol grids and effective when charged unidirectionally. in a given sense and in a predetermined amount to nullify the normal blocking action of said first control grid and thereby render said tube conductive, and means providing a circuit including an alternating current source and extending through a path in said flame of substantially unilateral conductivity and through said condenser whereby said condenser tends to become charged unidirectionally for rendering said tube conductive when a flame is present, a resistor connected in series with said condenser and eifective when said flame path is short-circuited to so limit the magnitude of the instantaneous alternating current charge on said condenser that said second control grid is ineffective to render said tube conductive, and a connection between the other side of said condenser and said cathode for causing said condenser to lose its unidirectional charge and thereby bec me ineffective when said circuit is interrupted due to flame failure.

5. In a flame detector operating in dependence upon the property of a flame to conduct electrical current between two spaced electrodes more readily when the first of said electrodes is positive with respect to the second electrode than when the relative polarities of said electrodes are reversed, the combination of an alternating current transformer having three taps, the potential of the second tap being intermediate the potentials of the first and th rd taps, an electron tive, means electrically connecting sad filament across said second and third taps to afford a heater circuit for said filament, a condenser electrically connected to the other of said control grids and adapted when charged to at least a "predetermined potential in a predetermined sense to bias said other control grid to overcome sad one control grid and thereby render said tube conductive, and means forming a flame detecting circuit including said condenser and a source of alternating current and the electrical path through the flame between said electrodes, and ground connections included in said flame detecting circuit and so disposed in said circuit as to said first'tap, means electrically connecting said cathode to said second tap, means electri- 4 cally connecting one of said control grids to said to induce a unidirectional charge of said predetermined sense on said condenser when a flame is present, and voltage control means also included in-said circuit and operable when said circuit is bilaterally conductive to so limit the magnitude of the instantaneous alternating charge on said condenser that sa d second control grid is rendered ineffective to render said tube conductive.

6. In a flame detector operating in dependence upon the property of a flame to conduct electrical current between two spaced electrodes more readily when the first of said electrodes is positive with respect to the second electrode than when the relative polarities of said electrodes are reversed, the combination of an alternating current transformer having a secondary prov'ded with three taps, the potential of the second tap being intermediate the potentials of the first and third taps, an electron tube having an anode, a cathode, a filament, and at least two control grids, means electrically connecting said anode third tap to negatively bias said one control grid to thereby render said tube normally nonconductive, a heater circuit for said filament connected across said second and third taps, a first resistor, a conductor connecting one end' of said first resistor to said first electrode, a condenser connected on one side to the other end of said first resistor and connected at its other side to the other of said control grids, electric circuit means including a source of alternating current coning circuit, whereby a biasing charge is built up in said condenser and said tube is rendered conductive only in response to the establishment of a substantially unidirectional charge on said condenser whenever said electrodes are electrically interconnected solelythrough the non-bilaterally conductive medium of a flame, and said flame-sensing circuit being so constructed and arranged that said tube becomes nonconductive upon flame failure as well as when a bilateral electrical connection exists between said electrodes, an inner sheath and an outer conduit surrounding at least a portion of said conductor in insulated relation tosaid conductor and to each other, means providing a ground connection for said conduit, and an electrical circuit means in eluding a resistor connected between said sheath and said other side of said condenser to afford a shunt circuit about said condenser in the event of insulation failure between said conduit and said sheath or said sheath and said conductor.

7. In a flame detector operating in dependence upon the property of a flame to conduct electrical current between two spaced electrodes more rent transformer having a secondary provided with three taps, the potential of the second tap being intermediate the potentials of .the first and third taps, an electron tube having an anode, a cathode and at least two control grids, means electrically connecting said anode to said first tap, means electrically connecting said cathode to said second tap, means electrically connecting one 01 said control grids to said third tap to negatively bias said one control and thereby render said tube normally nonconductive, a first resistor connected at one end to said first electrode, a condenser connected on one side to the other end of said first resistor and connected at its other side to the second one of said control grids, electric circuit means including a source of alternating current in phase with said transformer and connected betweensaid other side of said condenser and said second electrode to form a flame sensing circuit in which alternating current tends to flow in such phase relationship as to tend to bias said second control grid positive in those cycles when the anode is positive, whereby a biasing charge opposite to the biasing charge on said first control grid is built up in said condenser and said tube is rendered conductive only in response to the establishment of a substantially unidirectional charge on said condenser when second control grid is ineffective to cause conductivity of said tube, and said first resistor is efiective when a bilateral electrical connection exists between said electrodes to maintain the instantaneous positive bias on said second electrode below the value required to render said tube "anode, cathode and a first one of said control grids to block said tube and render the same non-conductive, a first resistor, a conductor connecting one end of said first resistor to said firstelectrode, a condenser connected on one side to the other end oi said first resistor and connected at its other side to the other of said control grids, electric circuit means including a, source of alternating current connected between said other side of said condenser and said second electrode to form a flame-sensing circuit, whereby a biasing charge is built up in said condenser and said tube is rendered conductive only in response to the establishment of a substantially unidirectional charge on said condenser whenever said electrodes are electrically interconnected solely through the non-bilaterally conductive medium of a flame, and said flame-sensing circuit being so constructed and arranged that said tube becomes non-conductive upon flame failure as well as when a bilateral electrical connection exists between said electrodes, an inner sheath and an outer conduit surrounding at least a portion of said conductor-in insulated relation to said conductor and to each other, means providing a ground connection for said conduit, and an electrical circuit means including a resistor connected between said sheath and said other side 01' said condenser to aflord a shunt circuit about said condenser in the event of insulation failure between said conduit and said sheath or said sheath and said conductor.

9. In a device for detecting the presence of a flame in a fuel burner or the like, an electron tube having an anode, a cathode and at least two control grids, means normally biasing said anode, cathode and a first one of said control grids to block said tube and render the same non-conductive, a condenser connected on one side to a second one of said control grids and effective when charged unidirectionally in a given sense and in a predetermined amount to nullify the normal blocking action of said first control grid and thereby render said tube conductive, and means providing a circuit including an altemating current source and extending through a path in said flame of substantially unilateral conductivity and through said condenser whereby said condenser tends to become charged unidirectionally for rendering said tube conductive when a flame is present, means in said circuit operable to limit the magnitude of the instantaneous alternating current charge impressed on said condenser and said second control grid in each cycle of the alternating current to a value insufficient to render said tube conductive whereby said second control grid is ineffective to render said tube conductive when said circuit is bilaterally conductive, and means connected to the other side of said condenser and afiording a leakage path for discharging said condenser of its unidirectional charge when the unidirectional condenser charging action of said circuit is discontinued due either to flame failure or to bilateral conductivity of said circuit.

ROBERT E. YATES. 

